This includes an important fix: Username and email must match or a valid
temporary password for any existing account could be sent to an arbitrary
unauthorized email address for capturing that account by a third person.

Now set_user_attribute() reveals it's potential for code cleanup as well.

Following user feedback the classic login form is the default again and CSS
style definitions for this template are not included but contributed
separately.

To get the new-style login form now you need to set login_opt_list = True
in your trac.iniand make sure, CSS styles for the login page
get loaded, since this is no longer done by AcctMgr itself.

I'm grateful for all the feedback given on the preliminary new-style login
form. Thank to all participants. A visible result is, that the "Register"
link is no longer moved but copied into the central login form box to double
the chance of being recognized by an unaware visitor.
Thanks especially to Mitar for some valuable template improvements, i.e.
for fixing XHTML compliance of the hidden field in login.html.

Plugin components can be activated implicitly, if the code is located in theplugins directory of the enironment. MethodComponentManager.is_component_enabled returns None for such a case,
so it's not at all a reliable method to detect component activation.

Special thanks to David Byrne for testing and for critical hints on the issue.

Build a property for RegistrationModule from the fixed _enable_check(),
further unify the way we refer to AccountManager(self.env), add/extend
Python doc strings, and shorten/simplify code in various places.